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#[[Orthorhombic]] γ-boron, containing 28 atoms per cell. |
#[[Orthorhombic]] γ-boron, containing 28 atoms per cell. |
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The γ-phase <ref |
The γ-phase <ref>Oganov A.R., Chen J., Gatti C., Ma Y.-M., Yu T., Liu Z., Glass C.W., Ma Y.-Z., Kurakevich O.O., Solozhenko V.L. "Ionic high-pressure form of elemental boron" [http://www.nature.com/nature/journal/v457/n7231/full/nature07736.html Nature 457 (2009) 863-867. Submitted 27 January 2007, published 28 January 2009]. [http://www.sciencedaily.com/releases/2009/01/090128215130.htm]</ref> is the densest (2.52 g/cm<sup>3</sup>) and the hardest (Vickers hardness 50 GPa)<ref>{V.L. Solozhenko, Kurakevych O.O.; Oganov A.R., On the hardness of a new boron phase, orthorhombic γ-B<sub>28</sub>. Journal of Superhard Materials 30 (2008), p.428-429</ref> known phase of boron. It can be produced by compressing other boron phases to 12-20 GPa, heating to 1500-1800 <sup>0</sup>C and is quenchable to ambient conditions and it is possible that this phase was observed back in 1965<ref>R.H. Wentorf. Boron: Another Form. Science 147, (1965), p. 49-50</ref>. Many characteristics of this phase are unusual, in particular its crystal structure <ref>Oganov A.R., Chen J., Gatti C., Ma Y.-M., Yu T., Liu Z., Glass C.W., Ma Y.-Z., Kurakevich O.O., Solozhenko V.L. "Ionic high-pressure form of elemental boron" [http://www.nature.com/nature/journal/v457/n7231/full/nature07736.html Nature 457 (2009) 863-867. Submitted 27 January 2007, published 28 January 2009]. [http://www.sciencedaily.com/releases/2009/01/090128215130.htm]</ref> that can be described as a NaCl-type arrangement of two types of clusters, B<sub>12</sub> icosahedra and B<sub>2</sub> pairs. Calculations suggest there is a significant [[charge transfer]] (~0.5 electrons) between these clusters, making γ-boron the only elemental solid (by 2009) with significantly [[Ionic bond|ionic]] type of bonding. Compressing boron above 160 GPa produces a boron phase with an as yet unknown structure, and this phase is a [[superconductor]] at temperatures 6-12 K <ref>M. I. Eremets et al. "Superconductivity in Boron" [http://www.sciencemag.org/cgi/content/abstract/293/5528/272?ck=nck Science 293 (2001) 272]</ref>. |
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Optical characteristics of crystalline/elemental boron include the transmittance of [[infrared]] light. At standard temperatures, elemental boron is a poor [[electrical conductivity|electrical conductor]], but is a good electrical conductor at high temperatures. |
Optical characteristics of crystalline/elemental boron include the transmittance of [[infrared]] light. At standard temperatures, elemental boron is a poor [[electrical conductivity|electrical conductor]], but is a good electrical conductor at high temperatures. |
Revision as of 05:43, 1 May 2009
Boron | ||||||||||||||||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Pronunciation | /ˈbɔːrɒn/ | |||||||||||||||||||||||||
Allotropes | α-, β-rhombohedral, β-tetragonal (and more) | |||||||||||||||||||||||||
Appearance | black-brown | |||||||||||||||||||||||||
Standard atomic weight Ar°(B) | ||||||||||||||||||||||||||
Boron in the periodic table | ||||||||||||||||||||||||||
| ||||||||||||||||||||||||||
Atomic number (Z) | 5 | |||||||||||||||||||||||||
Group | group 13 (boron group) | |||||||||||||||||||||||||
Period | period 2 | |||||||||||||||||||||||||
Block | p-block | |||||||||||||||||||||||||
Electron configuration | [He] 2s2 2p1 | |||||||||||||||||||||||||
Electrons per shell | 2, 3 | |||||||||||||||||||||||||
Physical properties | ||||||||||||||||||||||||||
Phase at STP | solid | |||||||||||||||||||||||||
Melting point | 2349 K (2076 °C, 3769 °F) | |||||||||||||||||||||||||
Boiling point | 4200 K (3927 °C, 7101 °F) | |||||||||||||||||||||||||
Density when liquid (at m.p.) | 2.08 g/cm3 | |||||||||||||||||||||||||
Heat of fusion | 50.2 kJ/mol | |||||||||||||||||||||||||
Heat of vaporization | 508 kJ/mol | |||||||||||||||||||||||||
Molar heat capacity | 11.087 J/(mol·K) | |||||||||||||||||||||||||
Vapor pressure
| ||||||||||||||||||||||||||
Atomic properties | ||||||||||||||||||||||||||
Oxidation states | −5, −1, 0,[4] +1, +2, +3[5][6] (a mildly acidic oxide) | |||||||||||||||||||||||||
Electronegativity | Pauling scale: 2.04 | |||||||||||||||||||||||||
Ionization energies |
| |||||||||||||||||||||||||
Atomic radius | empirical: 90 pm | |||||||||||||||||||||||||
Covalent radius | 84±3 pm | |||||||||||||||||||||||||
Van der Waals radius | 192 pm | |||||||||||||||||||||||||
Spectral lines of boron | ||||||||||||||||||||||||||
Other properties | ||||||||||||||||||||||||||
Natural occurrence | primordial | |||||||||||||||||||||||||
Crystal structure | rhombohedral | |||||||||||||||||||||||||
Thermal expansion | β form: 5–7 µm/(m⋅K) (at 25 °C)[7] | |||||||||||||||||||||||||
Thermal conductivity | 27.4 W/(m⋅K) | |||||||||||||||||||||||||
Electrical resistivity | ~106 Ω⋅m (at 20 °C) | |||||||||||||||||||||||||
Magnetic ordering | diamagnetic[8] | |||||||||||||||||||||||||
Molar magnetic susceptibility | −6.7×10−6 cm3/mol[8] | |||||||||||||||||||||||||
Speed of sound thin rod | 16,200 m/s (at 20 °C) | |||||||||||||||||||||||||
Mohs hardness | ~9.5 | |||||||||||||||||||||||||
CAS Number | 7440-42-8 | |||||||||||||||||||||||||
History | ||||||||||||||||||||||||||
Discovery | Joseph Louis Gay-Lussac and Louis Jacques Thénard[9] (30 June 1808) | |||||||||||||||||||||||||
First isolation | Humphry Davy[10] (9 July 1808) | |||||||||||||||||||||||||
Isotopes of boron | ||||||||||||||||||||||||||
| ||||||||||||||||||||||||||
Boron (Template:PronEng) is the chemical element with atomic number 5 and the chemical symbol B. Boron is a trivalent metalloid element which occurs abundantly in the evaporite ores borax and ulexite. Several allotropes of boron exist; amorphous boron is a brown powder, though crystalline boron is black, extremely hard (9.3 on Mohs' scale), and a weak conductor at room temperature. Elemental boron is used as a dopant in the semiconductor industry, while boron compounds play important roles as light structural materials, insecticides and preservatives, and reagents for chemical synthesis.
Boron is an essential plant nutrient, although high soil concentrations of boron may also be toxic to plants. As an ultratrace element, boron is necessary for the optimal health of rats and presumably other mammals, though its physiological role in animals is poorly understood.
Characteristics
Brown amorphous boron is a product of certain chemical reactions. It contains boron atoms that are randomly bonded to each other without long range order.
Crystalline boron, a very hard, black material with a high melting point, exists in four major polymorphs:
- Rhombohedral α-boron, containing 12 atoms in the unit cell.
- Rhombohedral ß-boron, containing 106.7 atoms per cell.
- Tetragonal boron, containing 192 atoms per cell.
- Orthorhombic γ-boron, containing 28 atoms per cell.
The γ-phase [11] is the densest (2.52 g/cm3) and the hardest (Vickers hardness 50 GPa)[12] known phase of boron. It can be produced by compressing other boron phases to 12-20 GPa, heating to 1500-1800 0C and is quenchable to ambient conditions and it is possible that this phase was observed back in 1965[13]. Many characteristics of this phase are unusual, in particular its crystal structure [14] that can be described as a NaCl-type arrangement of two types of clusters, B12 icosahedra and B2 pairs. Calculations suggest there is a significant charge transfer (~0.5 electrons) between these clusters, making γ-boron the only elemental solid (by 2009) with significantly ionic type of bonding. Compressing boron above 160 GPa produces a boron phase with an as yet unknown structure, and this phase is a superconductor at temperatures 6-12 K [15].
Optical characteristics of crystalline/elemental boron include the transmittance of infrared light. At standard temperatures, elemental boron is a poor electrical conductor, but is a good electrical conductor at high temperatures.
Boron compounds such as BCl3 behave as electrophiles or Lewis acids in their reactions. Boron is the least electronegative non-metal.
Boron is also similar to carbon with its capability to form stable covalently bonded molecular networks. Boron is also used for heat resistant alloys. Boron can form compounds whose formal oxidation state is not three, such as B(II) in B2F4.[16]
History and Etymology
Boron compounds were known thousands of years ago, but was never recognized as an element until Jöns Jakob Berzelius identified boron as an element in 1824. It was isolated by Sir Humphry Davy, Joseph Louis Gay-Lussac and Louis Jacques Thénard in 1808 through the reaction of boric acid and potassium.[17] The first pure boron was produced by the American chemist W. Weintraub in 1909, although this is disputed by some researchers.[18][19]
The name boron comes from the Arabic word buraq and the Persian word burah;[20] which are names for the mineral borax.[21]
Applications
Glass and ceramics
Nearly all boron ore extracted from the Earth is destined for refinement into boric acid and sodium tetraborate. In the United States 70% of the boron is used for the production of glass and ceramics. Borosilicate glass has a low coefficient of thermal expansion giving it a good resistance to thermal shock. Duran and Pyrex are two major brand names for this glass.
Soaps and detergents
Sodium perborate serves as a source of active oxygen in many detergents, laundry detergents, cleaning products, and laundry bleaches. It is also present in some tooth bleaching formulas.
Fire retardants
Zinc borate is used as fire retardant for plastics and rubber articles and also for cellulose insulation and in cotton mattresses.
Engineering materials
- Boron carbide, a ceramic material, is used to make armor materials.
- Boron nitride is a material isoelectronic with carbon, forming both graphite and diamond forms - it is used in similar applications - as a high temperature component and lubricant, and as an abrasive.
- Boron filaments are high-strength, lightweight materials that are chiefly used for advanced aerospace structures as a component of composite materials, as well as limited production consumer and sporting goods such as golf clubs and fishing rods.[22][23] The fibers can be produced by chemical vapor deposition of boron on a tungsten filament.[24][25]
- Magnesium diboride was discovered to become superconductive at temperatures below 39 K in 2001, MgB2 wires can be produced with the powder-in-tube (PIT) process.[26][27]
Boric acid
Boric acid (also known as otthoboric acid) H3BO3:
- Used in the production of textile fiberglass, flat panel displays[citation needed] and has use as a mild antiseptic.
- Traditionally used as an insecticide, notably against ants, fleas, and cockroaches.[28]
Sodium borates (Boraxes)
- Sodium tetraborate pentahydrate (Na2B4O7 • 5H2O), which is used in large amounts in making insulating fiberglass and sodium perborate bleach.
- Sodium tetraborate decahydrate (Na2B4O7 • 10 H2O) is used in the production of adhesives and in anti-corrosion systems.[citation needed]
- Used as a flux for soldering silver and gold and with ammonium chloride for welding ferrous metals.
- Borax is sometimes found in laundry detergent.
- Used as a water clarifier in swimming pool water treatment.
Other uses
- Triethylborane was used to ignite the JP-7 fuel of the Pratt / Whitney J-58 ramjet engines powering the Lockheed SR-71 Blackbird.
- Boron is an essential plant micronutrient.[29][30]
- Because of its distinctive green flame, amorphous boron is used in pyrotechnic flares.[31]
- Boron is used as a melting point depressant in nickel-chromium braze alloys.[32]
- Boron compounds show promise in treating arthritis.[33]
- Due to its high neutron cross-section, boron is often used to control fission in nuclear reactors [34]
Boron compounds with high hardness
- Heterodiamond (also called BCN) - compound of Carbon, boron, and nitrogen.
- Cubic boron nitride (CBN or borazon, the latter being the commercial name. Discovered in 1957).
- Rhenium diboride can be produced at ambient pressures, but is rather expensive because of rhenium. The hardness of ReB2 exhibits considerable anisotropy because of its hexagonal layered structure. Its value is much lower than that of diamond and is comparable to that of tungsten carbide, silicon carbide, titanium diboride or zirconium diboride [35].
- AlMgB14 + TiB2 composites possess high hardness and wear resistance and are used in either bulk form or as coatings for components exposed to severe environments [36].
Borides are used for coating tools (CVD or PVD). Boronized (or borided) metals and alloys, through means of ion implantation or ion beam deposition show a spectacular increase in surface resistance and microhardness. Laser alloying has also been successfully used for the same purpose.[citation needed]
These borides are an alternative to diamond coated tools, the treated surfaces have similar properties to the bulk boride. Methods that achieve greater depth of penetration boride are preferred over deposition methods since the borides are formed within the metallic substrate.[citation needed]
Boron compounds
The boron oxygen compounds sodium tetraborate, boric acid and sodium perborate are produced on a large scale. Sodium borohydride is one of the few hydrides produced on industrial scale as reduction reagent in chemical synthesis. For some special applications the carbide and nitride boron carbide and boron nitride are produced
Occurrence
The world wide commercial borate deposits are estimated to be 1010 kg of boron.[37][38] Turkey and the United States are the world's largest producers of boron.[25][39]Turkey has almost 72% of the world’s boron potential and boron reserves.[40] Boron does not appear on Earth in elemental form but is found combined in borax, boric acid, colemanite, kernite, ulexite and borates. Boric acid is sometimes found in volcanic spring waters. Ulexite is a borate mineral that naturally has properties of fiber optics.
Economically important sources are from the ore rasorite (kernite) and tincal (borax ore) which are both found in the Mojave Desert of California, with borax being the most important source there. The largest borax deposits are found in Central and Western Turkey including the provinces of Eskişehir, Kütahya and Balıkesir [41][42][43]
Production
Pure elemental boron is not easy to prepare. The earliest methods used involve reduction of boric oxide with metals such as magnesium or aluminum. However the product is almost always contaminated with metal borides. Pure boron can be prepared by reducing volatile boron halides with hydrogen at high temperatures. Very pure boron, for the use in semiconductor industry, is produced by the decomposition of diborane at high temperatures and then further purified with the Czochralski process.
Isotope enrichment
Due to the use of boron-10 in nuclear reactors as neutron-capturing substance, several industrial-scale enrichment process have been developed. Although nearly all possible enrichment methods are adaptable to boron enrichment, only the fractionated vacuum distillation of the dimethyl ether adduct of boron trifluoride (DME-BF3) and column chromatography of borates are used. [44]
Market trend
Estimated global consumption of boron rose to a record 1.8 million tonnes of B2O3 in 2005, following a period of strong growth in demand from Asia, Europe and North America. Boron mining and refining capacities are considered to be adequate to meet expected levels of growth through the next decade. The form in which boron is consumed has changed in recent years. The use of ores like colemanite has declined following concerns over arsenic content. Consumers have moved towards the use of refined borates or boric acid that have a lower pollutant content. The average cost of crystalline boron is $5/g.[45]
Increasing demand for boric acid has led a number of producers to invest in additional capacity. Eti Mine Company of Turkey opened a new 100,000 tonnes per year capacity boric acid plant at Emet in 2003. Rio Tinto Group increased the capacity of its boron plant from 260,000 tonnes per year in 2003 to 310,000 tonnes per year by May 2005, with plans to grow this to 366,000 tonnes per year in 2006.
Chinese boron producers have been unable to meet rapidly growing demand for high quality borates. This has led to imports of disodium tetraborate growing by a hundredfold between 2000 and 2005 and boric acid imports increasing by 28% per year over the same period.
The rise in global demand has been driven by high rates of growth in fiberglass and borosilicate production. A rapid increase in the manufacture of reinforcement-grade fiberglass in Asia with a consequent increase in demand for borates has offset the development of boron-free reinforcement-grade fiberglass in Europe and the USA. The recent rises in energy prices can be expected to lead to greater use of insulation-grade fiberglass, with consequent growth in the use of boron.
Roskill Consulting Group forecasts that world demand for boron will grow by 3.4% per year to reach 21 million tonnes by 2010. The highest growth in demand is expected to be in Asia where demand could rise by an average 5.7% per year.[46]
Boron in biology
A boron-containing natural antibiotic, boromycin, isolated from streptomyces, is known.[47][48]
Boron is an essential plant nutrient, required primarily for maintaining the integrity of cell walls. Conversely, high soil concentrations of > 1.0 ppm can cause marginal and tip necrosis in leaves as well as poor overall growth performance. Levels as low as 0.8 ppm can cause these same symptoms to appear in plants particularly sensitive to boron in the soil. Nearly all plants, even those somewhat tolerant of boron in the soil, will show at least some symptoms of boron toxicity when boron in the soil is greater than 1.8 ppm. When boron in the soil exceeds 2.0 ppm, few plants will perform well. Plants sensitive to boron in the soil may not survive. When boron levels in plant tissue exceed 200 ppm symptoms of boron toxicity are likely to appear.
As an ultratrace element, boron is necessary for the optimal health of rats, although it is necessary in such small amounts that ultrapurified foods and dust filtration of air is necessary to show the effects of boron deficiency, which manifest as poor coat/hair quality. Presumably, boron is necessary to other mammals. No deficiency syndrome in humans has been described. Small amounts of boron occur widely in the diet, and the amounts needed in the diet would, by analogy with rodent studies, be very small. The exact physiological role of boron in the animal kingdom is poorly understood.[49]
Boron occurs in all foods produced from plants. Since 1989 its nutritional value has been argued. It is thought that boron plays several biochemical roles in animals, including humans.[50] The U.S. Department of agriculture conducted an experiment in which postmenopausal women took 3 mg of boron a day. The results showed that supplemental boron reduced excretion of calcium by 44%, and activated estrogen and vitamin D. However, whether these effects were conventionally nutritional, or medicinal, could not be determined.
The US National Institute of Health quotes this source:
- Total daily boron intake in normal human diets ranges from 2.1–4.3 mg boron/kg body weight (bw)/day. [51]
Analytical quantification
For determination of boron content in food or materials the colorimetric curcumin method is used. Boron has to be transferred to boric acid or borates and on reaction with curcumin in acidic solution, a red colored boron-chelate complex, rosocyanine, is formed.
Isotopes
Boron has two naturally-occurring and stable isotopes, 11
B
(80.1%) and 10
B
(19.9%). The mass difference results in a wide range of δ11
B
values in natural waters, ranging from -16 to +59.[citation needed] There are 13 known isotopes of boron, the shortest-lived isotope is 7
B
which decays through proton emission and alpha decay. It has a half-life of 3.5×10-22 s. Isotopic fractionation of boron is controlled by the exchange reactions of the boron species B(OH)3 and B(OH)4. Boron isotopes are also fractionated during mineral crystallization, during H2O phase changes in hydrothermal systems, and during hydrothermal alteration of rock. The latter effect species preferential removal of the 10
B
(OH)4 ion onto clays results in solutions enriched in 11
B
(OH)3 may be responsible for the large 11
B
enrichment in seawater relative to both oceanic crust and continental crust; this difference may act as an isotopic signature.
The exotic 17
B
exhibits a nuclear halo. [citation needed]
Enriched boron (boron-10)
The 10
B
isotope is good at capturing thermal neutrons. Natural boron is about 20% 10
B
and 80%11
B
. The nuclear industry enriches natural boron to nearly pure 10
B
. The waste product, or depleted boron, is nearly pure 11
B
. 11
B
is a candidate as a fuel for aneutronic fusion and is used in the semiconductor industry. Enriched boron or 10
B
is used in both radiation shielding and in boron neutron capture therapy. In the latter, a compound containing 10
B
is attached to a muscle near a tumor. The patient is then treated with a relatively low dose of thermal neutrons. This causes energetic and short range alpha radiation from the boron to bombard the tumor.[52][53][54]
In nuclear reactors, 10
B
is used for reactivity control and in emergency shutdown systems. It can serve either function in the form of borosilicate control rods or as boric acid. In pressurized water reactors, boric acid is added to the reactor coolant when the plant is shut down for refueling. It is then slowly filtered out over many months as fissile material is used up and the fuel becomes less reactive.
In future manned interplanetary spacecraft, 10
B
has a theoretical role as structural material (as boron fibers or BN nanotube material) which also would serve a special role in the radiation shield. One of the difficulties in dealing with cosmic rays, which are mostly high energy protons, is that some secondary radiation from interaction of cosmic rays and spacecraft structural materials, is in the form of high energy spallation neutrons. Such neutrons can be moderated by materials high in light elements such as structural polyethylene, but the moderated neutrons continue to be a radiation hazard unless actively absorbed in a way which dumps the absorption energy in the shielding, far away from biological systems. Among light elements that absorb thermal neutrons, 6
Li
and 10
B
appear as potential spacecraft structural materials able to do double duty in this regard.
Depleted boron (boron-11)
Cosmic radiation produces secondary neutrons when it hits spacecraft structures. Neutrons cause fission in 10
B
if it is present in the spacecraft's semiconductors. This produces a gamma ray, an alpha particle, and a lithium ion. The resultant fission products may then dump charge into nearby chip structures, causing data loss (bit flipping, or single event upset). In radiation hardened semiconductor designs, one measure is to use depleted boron which is greatly enriched in 11
B
and contains almost no 10
B
. 11
B
is largely immune to radiation damage. Depleted boron is a by-product of the nuclear industry.
11
B
is also a candidate as a fuel for aneutronic fusion. When struck by a proton of about 500 keV, it produces three alpha particles and 8.7 MeV of energy. Most other fusion reactions involving hydrogen and helium produce penetrating neutron radiation, which induces long term radioactivity in reactor structures and weakens them, as well as endangering operating personnel. Whereas, the alpha particles from 11
B
fusion can be turned directly into electric power and all radiation stops as soon as the reactor is turned off.[55]
Boron in NMR spectroscopy
Both 10
B
and 11
B
possess nuclear spin. The nuclear spin of boron-10 is 3 and that of boron-11 is 3/2. These isotopes are, therefore, of use in nuclear magnetic resonance spectroscopy; and spectrometers specially adapted to detecting the boron-11 nucleus are available commercially. The boron-10 and boron-11 nuclei also cause splitting in the resonances of attached nuclei.
Precautions
Elemental boron is nontoxic and common boron compounds such as borates and boric acid have low toxicity (approximately similar to table salt with the lethal dose being 2 to 3 grams per kilogram) and therefore do not require special precautions while handling.
The boranes (boron hydrogen compounds) are toxic as well as highly flammable and require special care when handling. Sodium borohydride presents a fire hazard due to its reducing nature, and the liberation of hydrogen on contact with acid. Boron halides are corrosive.
See also
References
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